War, what is it good for?

I understand there are pros and cons to war, more specifically current wars, that is Afghanistan, Iraq, the war on terror and any other skirmishes that I, and others, may not be fully aware of.
I am asking people to contribute their ideas on war, be it pro or con, to see if we can come up with some kind of conclusion.
Is war worth it? What is the best approach? Do we invade to spread democracy (Iraq) or do we "plant the seed" (Libya) and nurture movements toward it?
Any other questions or information on war people can volunteer, would be great.
I personally think wars are a necessary evil but I am not completely sold on the concept. I mean, any war we get involved in must be for good reason right? Otherwise why do we sacrifice so much for them?
I truly respect and appreciate the soldiers for their work and though a person may not support a war they must always support the troops.
Thanks for your thoughts.

Conflict is the engine of human progress.
War is the greatest degree of conflict.
Should world peace ever break out, humanity will die out within a century of it’s institution.
But we shouldn’t worry about that since humanity cannot avoid conflict so long as we live on this Earth.

“Unlike the adaptors, who learn the point of cooperation fairly early on, manipulators battle on. And, once all lesser species have been overcome, they are so competitive and predatory that they are compelled to turn in on themselves. This nearly always evolves into tribal competition in one form or another and becomes more and more destructive – exactly like your own history. However this competition is vital to promote the leap from biological to technological evolution.

You need an arms race in order to make progress.

Your desire to dominate fuels a search for knowledge which the adaptors never acquire. And although your initial desire for knowledge is selfish and destructive, it begins the development of an intellectual self awareness, a form of higher consciousness, which never emerges in any other species. Not even while they are experiencing it, for example, can the intelligent adaptors – your dolphins – express the concepts of Love or Time.

Militarisation and the development of weapons of mass destruction are your first serious test at level one. You’re still not through that phase, though the signs are promising. There is no point whatsoever in my intervening to prevent your self-destruction. Your ability to survive these urges is a crucial test of your fitness to survive later stages. So I would not, never have and never will intervene to prevent a species from destroying itself. Most, in fact, do just that.”